Polyolefins such as ethylene/α-olefin copolymers are economically advantageous and are widely used in various molding methods such as extrusion molding or as resin modifiers. Although the ethylene/α-olefin copolymers have excellent properties, they are still desired to be improved in heat resistance, particularly heat aging resistance.
Under such circumstances, the present inventors have found that in the ethylene/α-olefin copolymers, a great number of double bonds remain at the ends of the copolymers, and the double bonds have influences on the heat resistance of the polymers. The present inventors have earnestly studied for the purpose of improving heat aging resistance with keeping the excellent properties, and as a result, they have found that an ethylene/α-olefin copolymer which is controlled to have a specific amount of double bonds in the copolymer main chain and has a specific density is an excellent ethylene/α-olefin copolymer capable of solving all of the above problems. Based on the finding, the present invention has been accomplished.
In Japanese Patent Laid-Open Publication No. 60632/1999, there is disclosed an ethylene polymer which is an ethylene homopolymer or a copolymer of ethylene and an α-olefin of 3 to 20 carbon atoms and has the properties: (A) the melt index at 190° C. under a load of 2.16 kg is in the range of 0.0001 to 1000 g/10 min, (B) the density is in the range of 0.90 to 0.985 g/cm3, (C) the molecular weight distribution, as measured by gel permeation chromatography, is in the range of 3 to 7, (D) the number of vinyl groups present at the molecular ends of the polymer is not more than 0.02 (vinyl group/1000 carbon atoms), (E) the number of vinylidene groups present at the molecular ends of the polymer is not more than 0.02 (vinylidene group/1000 carbon atoms), and (F) the zirconium content is not more than 10 ppb. This polymer, however, is not suitable as a impact strength modifier because of high density, and has poor transparency and poor heat-sealing properties.